Whole-body skeletal muscle mass: development and validation of total-body potassium prediction models

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Abstract
Background: A substantial proportion of total body potassium (TBK) in humans is found in skeletal muscle (SM), thus affording a means of predicting total-body SM from whole-body counter–measured 40K. There are now > 30 whole-body counters worldwide that have large cross-sectional and longitudinal TBK databases. Objective: We explored 2 SM prediction approaches, one based on the assumption that the ratio of TBK to SM is stable in healthy adults and the other on a multiple regression TBK-SM prediction equation. Design: Healthy subjects aged ≥ 20 y were recruited for body-composition evaluation. TBK and SM were measured by whole-body 40K counting and multislice magnetic resonance imaging, respectively. A conceptual model with empirically derived data was developed to link TBK and adipose tissue–free SM as the ratio of TBK to SM. Results: A total of 300 subjects (139 men and 161 women) of various ethnicities with a mean (± SD) body mass index (in kg/m2) of 25.1 ± 5.4 met the study entry criteria. The mean conceptual model–derived TBK-SM ratio was 122 mmol/kg, which was comparable to the measurement-derived TBK-SM ratios in men and women (119.9 ± 6.7 and 118.7 ± 8.4 mmol/kg, respectively), although the ratio tended to be lower in subjects aged ≥ 70 y. A strong linear correlation was observed between TBK and SM (r = 0.98, P < 0.001), with sex, race, and age as small but significant prediction model covariates. Conclusions: Two different types of prediction models were developed that provide validated approaches for estimating SM mass from 40K measurements by whole-body counting. These methods afford an opportunity to predict SM mass from TBK data collected in healthy adults.

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